1. Field of the Invention
The present invention relates to an irreversible optical device, and more particularly to an irreversible optical device utilizing an optical frequency shift, which device can carry out a phase shift and transmissivity modulation for light and a variation in the transmissivity of light depending on the wavelength of the light.
2. Description of the Prior Art
"Irreversible" means a variation in the effect which light undergoes, depending on the advancing direction of the light. Irreversible devices are used for optical communications, lasers and sensors to avoid a re-reflection of light and to guide a single-directional operation. Faraday rotors, isolators and circulators are known as representative irreversible devices. Typically, such devices utilize the Faraday effect to cause an irreversible operation.
The Faraday effect is a rotation phenomenon of the polarization plane of a beam of linearly polarized light when it passes through transparent matter such as lead or glass in a direction parallel to the direction of an applied strong magnetic field. The rotation direction of the polarization plane does not depend on the direction of propagated light, but depends on the direction of the magnetic field.
The above-mentioned Faraday devices, which cause an irreversible operation by utilizing the Faraday effect, require strong magnetic flux, a polarizer and a medium having a high magneto-optic constant. Typically, such Faraday devices use no optical fiber as a medium because the optical fiber has a very low magneto-optic constant. Where such Faraday devices are used in an optical fiber system, an insertion loss of several decibels occurs in the process of connecting those Faraday devices to optical fibers of the optical fiber system. In order to provide an optical fiber system involving a low insertion loss, an irreversible device consisting of optical fibers should be used.
In addition, it is difficult for known Faraday devices to perform different operations in accordance with different wavelengths of light and to appropriately modulate its operation in accordance with the wavelength of light.
An example of a conventional Faraday device is disclosed in International Publication No. WO9320475. That is, this patent discloses an irreversible phase shifter which is applied to an interferometer to obtain an irreversible transmissivity of light depending on the wavelength of the light. However, the irreversible phase shifter is constituted by a bulk-type Faraday device while exhibiting a larger insertion loss. Moreover, this device can not achieve a modulation of transmissivity.